2009PASP..121..428O

Flux calibrated emission-line images can be obtained with the Hubble Space Telescope's WFPC2 images through application of a series of steps. Corrections for the underlying continuum are necessary in some cases, as are corrections for nearby emission-lines that contaminate measurement of primary emission lines. However, the most important factor is usually the primary calibration coefficient, and this is best determined from in-orbit observations of extended nebula that have accurate ground-based flux calibrated observations. The secondary coefficients, which are only used to determine the small correction terms, can only be determined through use of the relative values of the camera characteristics determined prior to installation in orbit. Determination of the primary flux calibration coefficients have been made for WFPC2 filters using new observations of the Orion Nebula and the Ring Nebula. Primary and secondary calibration coefficients are given for the F469N, F487N, F502N, F656N, F658N, and F673N filters and their use in deriving flux-calibrated images from pipeline-processed images is described. There is no evidence for long-term variations in the calibration of these filters.